The present invention relates to telemetered security systems and, in particular, to a crystal controlled transmitter for transmitting pulse position encoded binary data from distributed system sensors, user input units or a system controller or central processing unit (CPU).
With increasing amounts of radio frequency transmissions, as well as emissions from various electronic equipment, over the years regulations have been developed in the U.S. and abroad for various classes of such equipment to establish acceptable radiation limits and broadcast ranges. For example, the European communities require the use of a crystal oscillator; whereas, the Federal Communications Commission in the United States has established definitive emission standards which must be met before a circuit may be released to market.
Although crystal oscillators provide frequency stability, by themselves they are not capable of operating at the required frequencies for the present application (i.e. 319.5 MHz), nor necessarily within the permitted emission limits. In designing such transmitters, it is also common practice to couple a lower frequency stage to one or more multiplier stages and/or amplifier stages to develop a desired carrier frequency. Appropriate modulation stages, in turn, are coupled as needed to the oscillator or amplifier stages. However, the resultant transmitter may still not meet applicable regulations, nor the present packaging goals.
One example Applicant is aware of where a crystal oscillator is used in a security system can be found in U.S. Pat. No. 3,848,231. There a crystal controlled transmitter of undisclosed construction is described which operates to transmit pulse position encoded messages of variable duration data frames and pulse widths, as opposed to the present fixed duration data frames and pulse widths. The crystal oscillator also operates to produce a frequency modulated output signal, whereas the present transmitter produces an amplitude modulated output.
Another reference Applicant is aware of is U.S. Pat. No. 4,581,606, which discloses a programmable sensor transmitter producing frequency shift key modulated signals that are transmitted by a 300 MHz transmitter. Again, though, no mention is made of the construction of the detailed transmitter or oscillator circuitry.